Quite a few of the openssl functions require a key or a certificate
parameter. PHP 4.0.5 and earlier have to use a key or certificate
resource returned by one of the openssl_get_xxx functions.
Later versions may use one of the following methods:
A string having the format
file://path/to/file.pem - the named file must
contain a PEM encoded certificate/private key (it may contain both)
A string containing the content of a certificate/key, PEM encoded
For private keys, you may also use the syntax
array($key, $passphrase) where $key represents a
key specified using the file:// or textual content notation above, and
$passphrase represents a string containing the passphrase for that
private key